This might be slightly OT: Since you can put a mp3 (and also mp2?) in a mp4 container, shouldn't it be possible to also do the same with a mpc-file? I've tried with Foobar2000 and it didn't work (which I kind of expected, otherwise that would have been done ages ago:-))

Theoretically it is possible, but why to do it ?
To use features .mp4 container provides ? (error resilance, streaming an so on)

I'm thinking portable players

You need a portable that is able to decode/play MPC stream anyway.
So, no point...

It is technically possible to extend MP4 format to store MPC data, just like Apple did with ALAC, but it would obviously require all software to have specifically added support for such files before being able to play them, same goes for hardware players, and with SV7 limitations it would completely miss the point (added container overhead, fast seeking would't work anyway because of SV7 cross-frame dependencies).

Ok, well put, thanks!